Sleep and other biological rhythm abnormalities often present serious difficulties to elderly humans. Despite this knowledge, very little attention has been paid to the general changes in biological rhythms which occur with age. A very few experiments have investigated the change in circadian periodicity or rate of phase shifting following a photoperiod shift, but all are either inadequately documented, inappropriately performed or contradictory. The present series of experiments would resolve these issues by systematic examination of the circadian rhythm system in the aging male golden hamsters and comparing the results to simultaneously gathered data in young hamsters. The studies will be able to demonstrate a) whether locomotor period length changes with age; b) whether entrainment rate or capacity change with age; c) whether the stability of the aging rhythm system in constant light is different from that in younger animals and d) whether phase relationships between the body temperature rhythm and the locomotor rhythm are different in aging hamsters. The experiments will incidentally generate data pertinent to rhythm changes which often precede death. Inasmuch as stone theorists have proposed that aging may be the product of loss of temporal organization among a variety of internal rhythms, it is important to understand the properties of such rhythms. With this research we can perform the basic studies pertinent to the use of drugs (such as lithium or antidepressants) to affect circadian rhythmicity and improve upon timing of wake and somnolence problems.